1. Technical Field
The present disclosure relates generally to the hydrogenation of aromatic hydrocarbons, and more particularly to apparatus and methods for heterogeneous catalytic hydrogenation of aromatic hydrocarbons. More specifically, the disclosure relates to the reduction of mass transfer limitations of such methods in the apparatus and methods of hydrogenating
2. Background of the Invention
Heterogeneous catalytic hydrogenation processes of various kinds are widely practiced on a commercial scale. Typical hydrogenation reactions are conducted at a pressure of from about 1 bar (100 kPa) to about 300 bar (30,000 kPa) and a temperature within the range of from about 40° C. to about 350° C. Examples include hydrogenation of aldehydes to alcohols and the hydrogenation of unsaturated hydrocarbons to saturated hydrocarbons.
Catalytic hydrogenation is in all the above cases a heterogeneous process. In designing a hydrogenation plant a chemical engineer has to decide whether the process is to be operated as a liquid phase process or as a vapor phase process. The former offers the possibility of a compact plant but often high operating pressures have to be used as a rate determining factor is usually the low solubility of hydrogen in the organic liquid phase. This means that the costs of plant construction and operation are significant factors in the overall process economics.
An example of hydrogenation of an unsaturated hydrocarbon is the production of cyclohexane from benzene. Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry and as a raw material for the industrial production of caprolactam and adipic acid, both of which are intermediates used in the production of nylon. On an industrial scale, cyclohexane is produced by reacting benzene with hydrogen. Typical catalysts for such hydrogenation reactions include Group VIII metal catalysts, such as nickel, palladium and platinum. This reaction is exothermic. The use of high temperatures is normally recommended so as to maximize conversion of benzene to cyclohexane, but isomerization of cyclohexane to methyl cyclopentane, which is extremely difficult to separate from cyclohexane, can occur.
Over the years, researchers have developed numerous processes for manufacturing cyclohexane from the hydrogenation of benzene. The majority of these processes differ from each other in the techniques used to compensate for impurities, found in either the reaction components themselves or that are generated during the hydrogenation process.
For example, U.S. Pat. No. 3,711,566 (Estes et al.) describes a process in which aromatic hydrocarbon feedstock containing sulfur are hydrogenated using a fluoridated platinum catalyst. Sulfur, a known poison to platinum catalysts, causes rapid deactivation of the catalyst as the hydrogenation process proceeds. Adding fluorine to the catalyst reduces sulfur poisoning; however, this undesirably increases hydrocracking activity that also deactivates the catalyst. Estes et al. inhibited hydrocracking activity by adding extremely small amounts, of carbon monoxide (a poison of metal catalysts itself) to the pure-hydrogen feed stream. This allowed the carbon monoxide to interact with the acidity of the fluoridated catalyst surface and prevent reactions, like hydrocracking, from taking place. Because carbon monoxide can also poison and deactivate the catalyst, care must be exercised in both purifying the hydrogen feed stream and in adding the carbon monoxide to the pure-hydrogen feed stream in order to achieve proper hydrogenation. This type of hydrogenation process therefore appears most useful when the hydrocarbon feedstock contains substantial amounts of sulfur requiring the catalyst to contain fluorine to prevent the sulfur from poisoning the catalyst.
U.S. Pat. No. 4,626,604 (Hiles et al.) describes a process in which hydrogenation occurs in a series of catalytic stages using at least three adiabatic reaction vessels. Because hydrogenation occurs in stages, lower operating temperatures can be used, which in turn reduces the formation of byproducts such as esters that can poison the catalysts and decrease, catalytic activity. However, Hiles et al. requires that the liquid unsaturated aromatic hydrocarbon be vaporized prior to mixing with the hydrogen gas. Portions of the vaporized unsaturated aromatic hydrocarbon are then hydrogenated in each catalytic stage before the saturated hydrocarbon is cooled and condensed back to liquid-form.
Accordingly, there is a need in industry for improved systems and processes for hydrogenating liquid aromatic hydrocarbons, whereby effective conversion is obtained. For example, systems and processes for enhanced conversion of benzene to cyclohexane are desired; such systems desirably do not lead to formation of a significant amount of undesirable cracking products, such as methylcyclopentane.